I've heard talk of a game idea I thought would be good. 'Mounted Midgets' where all PCs are a halfling or gnome or similar mounted on a dog or similar. With the game being set up to reduce issues of even small mounts, like needed rope climbing, etc. So as to allow some use of mounted combat feats and mounted combat scenarios. (Would be Pathfinder, but could be used in many other systems.)

I have a few games I've always got the itch to run, but haven't been able to free up the time and invest in writing it all out, so these are just rough ideas.

I have most of my ideas for Shadowrun, because the blend of magic and technology is just such a fascinating setting, and the criminal element means that there are a thousand difference scenarios you could run:

BTL dealer with a serial killer problem (a la Strange Days)

Decker/hacker with evidence, needs protection

Decker/hacker with evidence, needs to be eliminated

Insurance company pays runners to track down kidnappers in ransom case

Corporation hires runners to steal list of kidnap insurees from insurance company

Johnson hires runners to smuggle a large truck through Seattle to a specific destination

Johnson hires runners to hijack a truck, filming the resulting chaos

Johnson hires runners to clear a pirate clan off of a pacific island

Johnson hires runners to go after a leviathan (dragon of the sea)

Gang hires runners to deal with a street doc that 's doing work organlegging (I call this one 'chop shop')

High-ranking criminal hires runners to kidnap and hold a gambler's girlfriend during a high-stakes gambling tournament

... and I could go on. I always have more ideas for Shadowruns than I have time to play.

But I don't just have ideas for Shadowrun. I have a plethora of ideas;

D20 Modern Pirates game - no magic, just sails, crews, cutlasses and cannons. I have a whole setting worked up, and I've run a couple of real-life games as a test, but I don't have the time for the extra game, and there's no guarantee that it wouldn't blow up in my face as a game if I did try and run it on here.

D20 Future game - I have a few of these ideas, mostly along a specific setting (the one I used previously for my Bear & Robot game) http://www.myth-weavers.com/showthread.php?t=63766
Oddly, side detail, I was inspired to put that game together when I first heard about Bioware doing a space game- what became Mass Effect.

Some other inspiration comes from movies or video games, directly.
Via John Carter and Cowboys and Aliens, I had the idea for: Martian West
No one is really sure how the aliens came to Earth- but when the skies boiled with lightning, they did come. Red Martians and green monsters, squat men from Jupiter and mysterious women from Venus. There may be some that know what happened- Edison in his Menlo Park fortress, or Tesla in his Wizard's academy high in the mountains, but if they know they're not telling.

As near as anyone can tell, it's been about nine years since that fightful first night. Once the major cities were burned, the green aliens began fighting among themselves. As that died out, the survivors crawled out from their hiding places, and began to rebuild. Five years past the night of lighting, and the green monsters roze again, razing the land and killing all but the strongest. Again the factions fell to infighting, and again the survivors began to rebuild. There are some that worry the green martian monsters will rise again.

The British Empire has airship and clockwork mechanical men to keep them safe. There may be fighting on other continents, but those are no concern of the former American states- here, if you have a gun, you have a chance. Welcome to the West- what's left of it, at least.

Martian West was designed and run with D&D, using clasic D&D races in place of martians and others. We even had a warforged, hence the mechanical man, whom I acted as a Clockwork Bender. Edison has an organization of Knights (think Brotherhood of Steel) who take all technology for themselves. Counter to this, the Sorcerers of Tesla seemingly throw magic from their very bodies, casting fire and lightning from their hands. In between the factions, in the American Southwest, cowboys and indians still eek out a meager living, abandoning the ruins of the old world in favor of the new.

D&D - 4th edition - Reign of the God-Kings
I don't know where this idea came from, but I wanted to run a two-stage game. The first stage (completed) had a party of heroes going up against Orcus as he tried to usurp the place of The Raven Queen as the deity responsible for the dead. The heroes, epic level, fought through the hordes and finally confronted Orcus, slayed him, and were surprised as he murdered the Raven Queen with his dying breath, raising himself to a state of semi-deity. The heroes slew him, and gathered the shards of godhood that the Raven Queen had shed while dying- when the other gods approached them, demanding the shards.

The heroes did not like this- and Tiamat was the first to attack them. Armed with the shards themselves, they stood ready...

Stage 2 was going to be much MUCH later, as the new heroes grew up in the civilization created by the previous heroes. It's been 3,000 years, and the God-Kings (former heroes who refused to retire when they hit level cap) have ruled with an absolute iron fist. The old gods are dead, and the new Gods reign personally on the sundered realms of the Earth. Infighting among the former heroes means that each lives in isolation from the others. This would be a campaign to kill their former selves, and return the world to its natural order.

Airships! - I love airships. Stupid Hindeberg had to go ruin everything.
I have my game, Crimson Skies, yes based on the video games of the same name. I steal ideas as much as I make up my own. I want to ramp it up, have multiple crews all out there wandering and having adventures, but it's a lot of work and I'm only one very lazy GM.

Yeah, so there are my 'someday games.' Feel free to steal any ideas you like, or even take credit for them.